Calculate My Home
Units

Furnace Size Calculator

Enter the heated square footage, climate zone, insulation and furnace efficiency to get the output you need and the furnace input size to buy.

Units
sq ft

Finished, heated space only.

ft

The rules of thumb assume 8 ft; taller rooms scale up by volume.

IECC climate zones. Not sure? Your county's zone is on the DOE Building America map.

Insulation and age
%

On the furnace's yellow EnergyGuide label. An 80% furnace turns 80% of its input into heat.

Furnace for 1,800 sq ft in zone 4

100,000BTU/h input

The house needs about 85,500 BTU/h of heat. A 100k furnace at 96% delivers 96,000 BTU/h.
Heat delivered at 96% AFUE, by input size40k38,40060k57,60080k76,800100k96,000120k115,200needed 85,500 BTU/h

Swipe sideways to see the whole drawing.

Heat needed (output)
85,500BTU/h
Input for exactly that
89,063BTU/h
Gas at full fire
97cu ft/h
The math

  1. BTU per sq ft

    Zone 4: 45 to 50, middle = 47.5 BTU/h per sq ft

  2. Heat needed

    1,800 sq ft × 47.5 = 85,500 BTU/h

  3. Input needed

    85,500 BTU/h ÷ 0.96 AFUE = 89,063 BTU/h

  4. Nominal furnace

    smallest of 40k to 120k with input × 0.96 ≥ need = 100,000 BTU/h input

Range for this zone: 81,000 BTU/h (well insulated) to 90,000 BTU/h (leaky).

This is a floor-area rule of thumb. A Manual J heat loss calculation (ACCA) uses your walls, windows, air leakage and local winter design temperature, and it usually comes in lower for a tight house. Size to that report when you buy.

A 1,800 sq ft house in climate zone 4 with average insulation needs about 85,500 BTU/h of heat by the square-foot rule of thumb (47.5 BTU per sq ft). With a 96% AFUE furnace that takes 89,063 BTU/h of input, so the nominal size to price is a 100,000 BTU/h input furnace, which delivers 96,000 BTU/h. A Manual J heat loss calculation is what the final choice should rest on.

How the furnace size is calculated

Three steps: the heat the house loses on a cold day, the input that produces it, and the nearest furnace sold.

Heat needed (BTU/h) = heated sq ft × BTU per sq ft for the zone × (ceiling ÷ 8 ft)

Input rating = heat needed ÷ AFUE; buy the smallest nominal input whose input × AFUE covers the need

A furnace's big number is its input, the gas it burns per hour. AFUE says how much of that reaches the house: Bryant puts it as "a furnace with 80% AFUE delivers 80% of its input BTUs as heat." Output is always input × AFUE, which is why the same house needs a bigger 80% furnace than a 96% one.

BTU per square foot by climate zone

Climate zoneExamplesHeating BTU/h per sq ft
1South Florida, Hawaii30 to 35
2Most of Florida, south Texas35 to 40
3Central Texas, Georgia, the Carolinas, most of California40 to 45
4Virginia, Kentucky, Missouri45 to 50
5Ohio, Illinois, New York, Boston50 to 60
6 to 7Northern Midwest, northern New England50 to 60

Zones 1 to 5 are InspectAPedia's simplified heating table, built on the IECC climate zone map (the same zones as the DOE Building America guidance). For zones 6 and 7 we use Bryant's "cold" band of 50 to 60, since its table stops at 5. Both sources say newer, well insulated houses belong at the low end and older or poorly insulated houses at the high end; the insulation buttons pick the low end, middle or high end.

Worked example, and what moves it

Same 1,800 sq ft houseHeat neededInput neededFurnace to price
Zone 4, average, 96% AFUE85,500 BTU/h89,063 BTU/h100,000 (96,000 out)
Zone 4, average, 80% AFUE85,500 BTU/h106,875 BTU/h120,000 (96,000 out)
Zone 4, newer and tight, 96%81,000 BTU/h84,375 BTU/h100,000 (96,000 out)
Zone 5, average, 96%99,000 BTU/h103,125 BTU/h120,000 (115,200 out)

Nominal sizes are 40,000, 60,000, 80,000, 100,000 and 120,000 BTU/h input, the steps in the ENERGY STAR list of certified furnaces (Goodman's -040 to -120 models, for example). Bryant describes residential furnaces as running "from 40,000 to 120,000+ BTUs."

Ceilings over 8 ft scale the load up by volume: a 9 ft ceiling adds 12.5%.

What size furnace do I need for a 2,000 sq ft house?

In zone 4 with average insulation: 2,000 × 47.5 = 95,000 BTU/h of heat. At 96% AFUE that is 98,958 BTU/h of input, and a 100,000 BTU/h furnace (96,000 out) covers it with about 1% to spare. With an 80% furnace the input climbs to 118,750 BTU/h and the size to 120,000. In zone 5 the same house needs 110,000 BTU/h at the middle of the band, beyond what a 100,000 furnace delivers at any AFUE. Bryant's own range for 2,000 sq ft is 60,000 to 120,000 BTU/h depending on insulation and climate.

Why not simply round up

A furnace that is far too big short-cycles: it heats the air in a few minutes, shuts off, and repeats, with more wear and uneven rooms. It also needs more airflow than a small duct system can carry. When the rule of thumb sits right at a size boundary, the smaller furnace plus a Manual J is usually the better bet than the bigger one.

Gas supply and ducts

At full fire, a 100,000 BTU/h input burns about 97 cubic feet of natural gas an hour (1,036 BTU per cubic foot, EIA). Check the line with the gas pipe size calculator, and size the supply ducts for the blower's airflow in the ductwork calculator.

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